Nuclear Science and Techniques

Abstract:

Phosphorylurea molecules, which contain both phosphoryl and carbonyl groups, are considered efficient extractants for UO₂²⁺ . This study aims to explain the complexation of UO₂²⁺ with carbamoylphosphoramidic acid (CPO), a simple model for phosphorylurea, for ligand design for uranium recovery from seawater using density functional theory calculations, natural bond order analysis, and the quantum theory of atoms in molecules. The results showed that, when CPO acts as a monodentate ligand, the affinity of phosphoryl for UO₂²⁺ is stronger than that of carbonyl, and CPO coordinates with UO₂²⁺ through the phosphoryl oxygen atom. When CPO serves as a bidentate ligand, both the phosphoryl and carbonyl oxygen atoms connect to UO₂²⁺, and the U–O(carbonyl) bond plays a more important role than the U–O(phosphoryl) bond in the interaction between UO₂²⁺ and CPO. This paradox may be caused by the significant charge transfer from the U– O(carbonyl) π bond orbital to the C–N σ antibond orbital of the bidentate CPO. The NH spacer between the phosphoryl and carbonyl groups could ensure the delocalization of the electron system of the molecule. The bidentate binding motif is favored by entropy and opposed by enthalpy, while the monodentate binding motif is favored by enthalpy and opposed by entropy. Ultimately, the bidentate binding motif is more favorable than the monodentate one. As expected, the interaction between UO₂²⁺ and the deprotonated CPO is stronger than that between UO₂²⁺ and the neutral CPO. Comparing the interaction between UO₂²⁺ and CPO with that between UO₂²⁺ and Nphenylcarbamoylphosphoramidic acid (PhCPO), formed by replacing one hydrogen atom from the terminal nitrogen atom of CPO with a phenyl group, the phenyl substituent at the terminal nitrogen atom of PhCPO shows a slightly negative effect on the interaction between UO₂²⁺ and PhCPO.

Key words: Uranium extraction, Adsorption, Density functional theory